This invention relates to inverters powered from a current source. More specifically, the invention relates to a DC-to-DC converter in which the inverter and saturating output transformer of a Royer circuit are powered from a current source rather than from a voltage source.
DC-to-DC converters, commonly known as "switchers" or "switching converters", find wide use in applications requiring DC voltage outputs which are well-regulated and substantially ripple free. Well-designed converters are capable of providing such output voltages over a wide range of DC inputs and varying load requirements. In general, such converters operate by converting the DC input to AC, transforming the AC to a higher or lower voltage as desired, and rectifying the AC to provide the required DC output voltage. It is also common in such converters to regulate the output voltage by means of one or more feedback loops. Typically, in response to a feedback signal the duty cycle of a switch, usually a power transistor, is varied, thereby providing the necessary regulation. The term pulse width modulation (PWM) is frequently used to describe such control strategy. The circuit portion of the DC-to-DC converter which provides the AC input to the rectifier circuit is known as an inverter, although the term converter is also used. The AC output of the inverter can be regulated against variations in the DC input or load requirements and employed to supply power to AC loads.
Cronin et al. U.S. Pat. No. 4,034,280, issued July 5, 1977 (incorporated herein by reference) discloses what may now be referred to as a conventional current powered DC-to-DC converter, in comparison to which the advance of the present invention may best be appreciated. In the patented device, a current source, comprising an inductor, provides essentially constant current to a push-pull inverter coupled to the primary winding of a linear transformer. A square wave source drives the inverter and synchronizes the operation of a digital control signal processor (DCSP). The primary function of the DCSP unit is to pulse width modulate, in response to a feedback signal, a transistor switch disposed in the inverter's positive input line and in series with the inductor.
Several drawbacks are associated with the above-described converter. The inverter utilized in the described device is known as a driven linear inverter. The inverter is termed linear because its output transformer is operated below the saturation point. Inverters of this type require a separate source of inverter drive such as, for example, a square wave generator. Linear operation of the inverter is acquired at the expense of increased output transformer size and added drive circuitry, and consequently at greater weight and cost. The connection of the PWM power transistor in series with the inductor in the positive (high) line of the DC input introduces additional complexity into the design. Such connection of the PWM power transistor requires that the low voltage drive output of the DCSP unit be isolated from the floating high positive voltage potential of the power transistor's base-emitter junction. Transformer isolation of the low voltage PWM drive is more difficult to achieve than isolation of the square wave inverter drive since additional demands are placed on the design of the transformer due to the non-symmetrical drive of the PWM power transistor (i.e., the duty cycle deviates from 50 percent). For example, if the drive is on for 90 percent and off for 10 percent, or vice versa, provisions must be made in the magnetic design to accommodate the volts x seconds unbalance to avoid saturation of the transformer. In contrast, in the square wave inverter the duty cycle does not depart from 50 percent so that the flux is symmetrical in both directions, allowing a simplified transformer design.
The present invention results in circuit simplification compared to the type of converter described above and additionally avoids the transistor current overshoot inherent in a Royer circuit operated from a constant voltage source.
In accordance with the invention, significant reduction in circuit complexity is realized by isolating the square wave inverter of the Royer circuit, rather than the low voltage PWM drive for the PWM power transistor as described above. To this end, the emitter of the PWM power transistor, in the inverter return line, is connected so there is a low voltage potential between it and the common DC supply return. The pulse width modulation signal for the PWM power transistor switch is referenced with respect to the common supply return. This allows the PWM power transistor to be direct-coupled from the low voltage PWM signal regulator. Isolation of the inverter in the manner described also simplifies the magnetic design since, as discussed above, the necessity to account for non-symmetrical drive is eliminated. It should be additionally noted that isolation of the inverter and the connection of the emitter of the PWM power transistor so that a low voltage potential exists between it and the common terminal of the DC supply, similarly permits simplification of the magnetic design of a driven inverter.
Other significant improvements and circuit simplifications are derived from employing a Royer circuit powered from a current source. Since the Royer circuit utilizes a saturating transformer, the inverter, once started, requires no separate source of drive for commutating the inverter transistors. The energy stored in the transformer magnetic field, along with properly phased drive windings, is used to provide the bias for the inverter transistors. Furthermore, powering the Royer circuit from a current source limits the current that can flow at the shut-off of one inverter transistor and the start-up of another. As a result, the current overshoot of 100 percent to 500 percent present in the constant voltage sourced Royer circuit is reduced to less than 10 percent with the current sourced Royer circuit.
These and other improvements provided by the present invention will be more fully described in the detailed description of the invention.